The invention relates to a monitoring device for monitoring lamps, especially in motor vehicles, comprising a measuring resistor connected in series with the incandescent lamp to be monitored and means for monitoring the potential drop at the measuring resistor.
The proper operation of the illumination in motor vehicles is of major importance to the security of the vehicle. Monitoring devices of the kind stated above have already been proposed for monitoring the operation of all essential illumination elements during driving. Regarding reliability, a high standard is requested from these monitoring devices. On the other hand, it is required that these monitoring devices may be produced at low costs, as otherwise they would not achieve the desired divulgation in as many motor vehicles as possible. Particular demands on a monitoring device of this kind are the following:
(a) an erroneous indication indicating that a defective lamp is still operating must be excluded;
(b) the readiness of operation of the monitoring device must be recognizable by simulating errors;
(c) a safe indication of malfunctions must be insured even in case of fluctuations from minus 20 to plus 25% of the voltage in the circuit of the motor vehicle;
(d) a maximum of 2.5% of the supply voltage may drop at the incandescent lamp to be monitored in the monitoring device;
(e) in spite of the relatively large resistance tolerances of incandescent lamps a safe indication of malfunction must be insured;
(f) in case of two parallel incandescent lamps to be monitored, when the total current to be monitored drops to 78% (with respect to the nominal power at testing voltage), no malfunction should be indicated;
(g) the safe indication of malfunction should be insured within a range of minus 40.degree. to +90.degree. C. of environment temperature;
(h) with an overload of the monitoring circuit with the 25 fold nominal current, a maximum of 25A, during 5 minutes, the device must still operate or become defective and indicate a malfunction.